How to disable barriers?

Gaia asked:

It seems impossible to disable barriers. I have tried loading the kernel with “mount -o barrier=off”, “mount -o barrier=0”, “barrier=off” and “barrier=0”. The kernel is Atomic Secure Linux 2.6.32 with CentOS 6.2 (now 6.3). Anywhere from 20 min to 4 hr after reboot I get an “kernel: JBD: barrier-based sync failed on vda1-8 – disabling barriers” in /var/log/messages.

>cat /etc/fstab
/dev/mapper/vg_centos62-lv_root  /  ext4  grpquota,usrquota,rw  0  1
UUID=bb756d7c-5946-4608-bec5-08411daa875e /boot                   ext4    defaults        1 2
/dev/mapper/vg_centos62-lv_swap swap                    swap    defaults        0 0
tmpfs                   /dev/shm                tmpfs   defaults        0 0
devpts                  /dev/pts                devpts  gid=5,mode=620  0 0
sysfs                   /sys                    sysfs   defaults        0 0
proc                    /proc                   proc    defaults        0 0

>cat /etc/mtab
/dev/mapper/vg_centos62-lv_root / ext4 rw,grpquota,usrquota 0 0
proc /proc proc rw 0 0
sysfs /sys sysfs rw 0 0
devpts /dev/pts devpts rw,gid=5,mode=620 0 0
tmpfs /dev/shm tmpfs rw 0 0
/dev/vda1 /boot ext4 rw 0 0
none /proc/sys/fs/binfmt_misc binfmt_misc rw 0 0
/tmp /tmp none rw,bind 0 0
/var/tmp /var/tmp none rw,bind 0 0
/home /home none rw,bind 0 0

I was also told that I have a loopback mount listed in /etc/mtab for /home to itself. I dont know if this could be the cause of it or if it is bad at all. This machine is using KVM virtualization and its disks reside on the host as .img files in an LVM.

My answer:

This is an issue with LVM in kernels prior to 2.6.33, where barrier support was completely implemented (the implementation in 2.6.32 was incomplete).

The error message is harmless and can be ignored (presuming your underlying disk array has battery backup or similar write cache protection).

You can also put the nobarrier option in /etc/fstab for this filesystem, which will suppress the message.

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